Organic light emitting diodes (OLEDs), as current-driven light emitting devices, have been applied to high performance displays more and more. With the increase of size of a display, a traditional passive matrix OLED display requires a shorter time period to drive an individual pixel and thus transient current needs to be increased and power consumption will be increased. Meanwhile, application with large current may cause too large voltage drop on ITO lines, resulting in a too high operation voltage of the OLED and a low efficiency thereof. An active matrix OLED display can solve these problems well by inputting currents into the OLEDs in a progressive scanning manner by switching transistors.
In the design of the AMOLED backplane, a main problem to be solved is the non-uniformity in brightness between pixels.
An AMOLED voltage type pixel unit driving circuit is provided in the prior art. Compared with the traditional AMLCD driving method, in the voltage type pixel unit driving circuit, a voltage signal representing a grey scale is provided by the driving unit, the voltage signal can be converted into a current signal for driving a transistor in the pixel circuit so as to drive the OLED to realize gray scale in brightness, this method can lead to a fast driving speed and can be realized simply, is suitable to drive a large-sized panel, and thus it is widely adopted in the art. However, it is required to design an additional TFT and a capacitor to compensate for the non-uniformity of TFT, IR Drop and non-uniformity of OLEDs.
FIG.1 shows a circuit structure of a most traditional voltage-driven pixel unit consisting of two TFTs and a capacitor (2T1C). The switch transistor TK transmits a data voltage Vdata, representing a gray scale, on the data line to a gate of the driving transistor TQ, the driving transistor converts the data voltage Vdata into a corresponding current to supply to the OLED, during a normal operation, the driving transistor TQ should be in a saturation region and supplies a constant current during a scan period for a row. The operation current of the driving transistor TQ, that is, the operation current of the OLED, may be represented as:
      I    OLED    =            1      2        ⁢                  μ        n            ·      Cox      ·              W        L            ·                        (                      Vdata            -            Voled            -            Vthn                    )                2            
where μn is a carrier mobility, Cox is a capacitance of a gate oxide layer, W/L is a width-length ratio of the transistor, Vdata is the data voltage, Voled is an operating voltage of the OLED and is shared by all pixel units, Vthn is a threshold voltage of the pixel unit (that is, the threshold voltage of the driving transistor TQ), for an enhanced-mode TFT, Vthn thereof is positive, and for a depletion mode TFT, Vthn thereof is negative.
Although the driving circuit of the pixel unit in the prior art is widely used, the following problems are still unavoidable: if different pixel units are different in Vthn, currents of the OLEDs are certainly different. If the Vthn of the pixel unit is drifted over time, currents of the OLED after and before the drift of the threshold may be different, leading to ghost. In addition, difference in the operating voltages of the OLEDs caused by non-uniformity of the OLEDs may result in different operating currents of the OLEDs.